Spread spectrum communications have been established as one of the more viable alternatives for cellular systems. In such systems, the particularly desirable property of low cross-correlations of PN spread spectrum codes for signals separated in time by an amount equal to or larger than one code element (chip), has given rise to the use of the same PN code in the same frequency band by different base stations in order to maximize capacity and facilitate system engineering. A PN code is a binary pseudorandom code and each of its elements is called a chip. Several well known sets of PN codes exist, such as M sequences and Gold codes. Because of the pseudonoise code properties, the code autocorrelation results a peak only when the offset between the received and the local PN codes is less than a chip. In that case, the received and local codes become effectively synchronized.
Signal separation among different cells can be achieved by having the different base stations use different offsets of the same PN code. The user station can simultaneously demodulate signals that are strong enough to be discriminated from background noise and are separated by more than one chip in time. Therefore, the user station, which is typically mobile, can simultaneously maintain communication with more than one base station, to enable smooth transitions between base stations. This concept is known in the literature as soft hand-off. To facilitate soft hand-off, the user station may identify received pilot multipaths from several base stations that use the same PN code but have an appropriately different PN code offset, measure the combined SNR of multipaths corresponding to a pilot signal transmitted by each of the base stations, and report the pilot PN code offsets and the SNR measurements to the base station or stations with which it currently communicates.
Since cell sites can have different sizes and the user station can be at any point within a particular cell, the propagation delay of the pilot transmitted by a base station can be on the order of a few tens to a few hundred chips. Once a user station has established communications with at least one base station, that base station communicates to the user station certain predetermined parameters relating to information about the cellular system. Among those parameters is timing information, in terms of PN code offsets from the base station's pilot offset, for PN codes of pilots corresponding to other base stations. Those base stations, and the corresponding pilots, are categorized into sets depending on the likelihood for the base station to participate in soft handoff.
For each pilot, the propagation delay range establishes a PN code offset uncertainty area around the PN code offset with which the pilot was originally transmitted by the corresponding base station. The user station needs to examine the entirety of such PN code offset uncertainty area, determine each offset at which a path with an SNR above a specified level exists, accurately measure the combined SNR for the possibly more than one paths originating from the pilot signal that is transmitted by each base station, and report the results to the base station or stations currently supporting the communication.
The user station needs to maintain pilot signals in sets, called soft hand-off sets. The set of each pilot from a particular base station is primarily determined by the corresponding SNR. There are usually several pilot signals in each set and each of them needs to have its strength and relative offset determined within a set-dependent time period. Pilot sets that are more important for reliable communications need to be updated more frequently. For proper performance, excessive delays between successive SNR measurements must be avoided. Thus, there is a need to minimize the time required to reliably identify the offsets corresponding to usable pilot signals. Only for those offsets are subsequent time consuming SNR measurements performed. Another reason for minimizing the time required for identification of usable pilot paths is to limit power consumption at the user station.
Finally, in addition to soft handoff, the user station needs to identify offsets corresponding to paths with the largest SNRs in order to assign those paths to available demodulators, also known as Rake fingers, and maximize the total SNR in order to optimize the communication quality. Although paths corresponding to the information signal need to be identified for that purpose, if the transmission of signals from the base station is synchronous, identifying a pilot path is equivalent to identifying an information signal path. This goal also requires continuous search of a window for path identification and SNR measurements.